1. Field of the Invention
This invention relates to gas turbine engines, and more particularly, to a diffusion barrier layer applied to airfoils in the turbine portion of a gas turbine engine.
2. Discussion of the Prior Art
Coatings, such a diffusion aluminides and MCrAlX overlays, are used on airfoils exposed to hot combustion gases in gas turbine engines to protect them from oxidation and corrosion attack, and to function as bond coats in thermal barrier coating (TBC) systems. These coatings are applied over substrate materials, typically nickel-based superalloys, on the surfaces directly exposed to the environment by techniques such as thermal spray, electron beam physical vapor deposition (EB-PVD), magnetron sputtering, cathodic arc, and other physical deposition techniques. In the case when TBC systems are being created, an additional coating of a thermally resistant ceramic coating, such as yttria-stabilized zirconia (YSZ) is applied over the bond coat. TBC coatings are typically applied by EB-PVD or thermal spray techniques.
The MCrAlX designation for the overlay coating generically describes a variety of chemical compositions that may be employed as environmental coatings or bond coats in TBC systems. In this and other forms, M refers to an element consisting of Ni, Co and Fe, or combinations thereof. X denotes elements selected from the group consisting of Ta, Re, Ru, Pt, Si, B, C, Hf, Y, Pt and Zr and combinations thereof. Recent developments in such overlay coatings have identified NiAlCrZr and NiAlZr bond coats as providing significant benefits in TBC spallation resistance over certain baseline TBC systems. Preferred composition ranges for NiAlCrZr and NiAlZr are described in pending application Ser. Nos. 09/166,883 (filed Oct. 6, 1998); 08/932,304 (filed Sep. 17, 1997, and CIP 09/232,518 (filed Jan. 19, 1999), respectively, assigned to the assignee of the present invention and incorporated herein by reference.
Service exposure of components with these environmental coatings (including TBC systems) under the hot, oxidative, corrosive environment causes a number of metallurgical processes to alter the airfoil system. Initially, the aluminum-rich coating forms a highly adherent thermally grown oxide (TGO) layer which grows at the interface between the diffusion aluminides or MCrAlX overlay bond coats and ceramic coatings. With further high temperature service exposure, spallation of the YSZ topcoat occurs at either the bond coat/alumina interface or at the alumina/YSZ interface. Although many factors influence the spallation performance of the TBC systems, it is clear that the interdiffusion of elements, which modifies the local chemistries of the substrate, bond coating and TGO, plays a major role in degrading the system. Essentially, there is a tendency for aluminum (Al) from the aluminum-rich overlay coating to diffuse into the substrate, while traditional alloying elements, such as Co, Cr, W, Re, Ta, Mo, and Ti present in the superalloy, migrate from the substrate into the coating as a result of compositional gradients. The depth of interdiffusion and extent of elemental change will depend on chemistry and temperature. The diffusional loss of Al to the substrate reduces the concentration of Al in the overlay coating, thereby reducing the ability of the overlay coating to continue generation of highly protective and adherent alumina scale. Simultaneously, the migration of the aforementioned superalloy elements likely affects the protective properties of the alumina. Another result of interdiffusion of Al and coating elements is the formation of a diffusion zone into the airfoil wall which results in the undesirable consumption of the load bearing airfoil wall.
What is needed is a diffusion barrier between the overlay coating and the substrate alloy that prolongs coating life by extending the time the coating chemistry provides a protective and adherent alumina scale, while being essentially chemically stable, being in contact with the bond coat and the superalloy, and highly adherent to both the substrate superalloy and the bond coat. In addition, the diffusion barrier should have low solubility and interdiffusivity with elements from the substrate and coating, and be easily deposited. The diffusion barrier should be as thin as possible to minimize the amount of weight added to the system. Intermetallics and solid solution alloys having compositions that are thermodynamically stable and that do not readily combine with aluminum to form new phases are likely candidates for diffusion barriers. These metallic materials are described in pending application Ser. No. 09/275,096 filed Mar. 24, 1999, assigned to the assignee of the present invention. The thicknesses of these metallic alloys may be somewhat greater than is otherwise attainable utilizing ceramic layers and may permit some diffusion of aluminum, either through the matrix or, in some circumstances, along the grain boundaries. Nevertheless, these metallic materials represent one potential solution to the problem of extending the life of airfoil coating systems. However, a very thin coating of a non-metallic material that is substantially impermeable to aluminum is another solution.
The present invention is directed toward a thin diffusion barrier layer for use as an intermediate layer between a nickel-base superalloy substrate, and in its broadest embodiment, an outer aluminum-containing layer. The diffusion barrier layer is a ceramic, such as an oxide, that forms a tightly adherent, thin film which is compatible with a nickel-base superalloy. The xe2x80x9cbarrierxe2x80x9d characteristics of this film is based on the reduced ability of aluminum from the bond coat and elements from the substrate to diffuse through it at elevated temperatures. Another aspect of the invention is that the ceramic diffusion barrier layer is applied to the substrate in such a manner so as to be both chemically bonded and optionally mechanically bonded to the substrate.
The airfoil of the present invention provides a system that increases the life expectancy of the airfoil by increasing the spallation resistance of applied coatings and minimizing the effects of interdiffusion, the system comprising of a substrate material, a thin ceramic layer chemically and optionally mechanically bonded to the substrate layer. A protective overlay coating that includes aluminum is applied over the thin oxide layer, the overlay coating being at least chemically bonded to the thin oxide layer and optionally mechanically bonded to the thin oxide layer, the overlay coating including an outer protective scale of alumina, and, when the overlay coating is used as a bond coat, an optional thermal barrier coating overlying the alumina. When no thermal barrier coating is applied, the overlay coating having the protective alumina scale forms a protective environmental layer.
An advantage of the present invention is that it slows down the growth of a diffusion layer from an outer aluminum-containing layer into a nickel-base substrate material. Thus, the airfoil maintains a substantial portion of its wall thickness so that, during repair, material removal is reduced as the diffusion zone is thinner. This in turn means that the airfoil can undergo multiple repair cycles.
Another advantage of the system of the present invention is that the diffusion barrier layer reduces the loss of aluminum, a critical scale-forming element, by inhibiting the inward migration of aluminum from the outer protective coating to the lower-aluminum containing substrate. As a result, the oxidation and corrosion resistance of the coating is maintained when the coating is used as an environmental coating. The adherence of the ceramic top coat is maintained when the coating is used as a bond coat leading to longer mean life between repairs. Additionally, the diffusion barrier also advantageously retards or prevents the outward migration of one or more substrate elements into the coating during high temperature operation. It is believed that the outward migration of some of these elements adversely affects the protective properties of the alumina scale.
The present invention provides for an article for use in a high temperature oxidative environment comprising a nickel-based superalloy substrate. Overlying the nickel-based superalloy substrate is a tightly adherent layer that acts as a diffusion barrier. The diffusion barrier layer is an intermediate coating between the substrate and an outer coating having a high concentration of aluminum. Typically, the overlay coating is designated as MCrAlX. The ceramic diffusion barrier layer ideally is a tightly adherent ceramic such as an oxide having thermodynamic stability at or above the temperatures of operation, a low diffusion permeability, low solubility for Al from the coating and ideally low solubility for refractory elements such as W, Ta, Mo, Re and other elements such as Ti and Co typically found in the substrate. Finally, the diffusion barrier layer is sufficiently bonded to both the outer protective coating and the superalloy to survive the high temperature atmosphere and thermal fluctuations experienced in a gas turbine engine.
An advantage of such diffusion barriers is that they are typically limited to less than a few microns in thickness so that it can be applied without adversely affecting the weight of the airfoil. Weight added to blades rotating at high speeds has a detrimental effect on stresses, which in turn has a detrimental effect on fatigue life.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention.